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A photo-desmear method for via residue removal using a VUV light source

Keywords: Desmear, Environmentally Friendly, Ultra Violet Light

The desmear process of removing via residue after laser drill in substrate manufacturing is a critical step for reliability and electrical performance. Cost effective and environmentally clean desmear processes are industry-wide objectives, as the current wet desmear process results in significant chemical waste and regular bath replacement.
Here, we evaluated an effective photo desmear method targeting a low total cost of ownership and negligible environmental impact. This method achieves residue and silica free via bottoms with the use of 172nm vacuum ultraviolet light (VUV) photo-chemical ashing followed by water clean.
Studies of organic molecular photochemical decomposition by short wavelength UV light are widely reported and have been applied for purposes such as surface cleaning and improving wettability and are naturally extendable to via residue removal. Scanning electron microscope (SEM) and energy dispersed spectroscopy (EDS) reveal that the application of VUV desmear results in residue free microvia without negatively affecting via shape or the dielectric surface. In addition, the surface remains highly wettable, which aids downstream copper plating. The high reaction rate, absence of wet chemistry, and creation of surfaces amenable for subsequent copper plating make this dry desmear process a strong candidate for future applications in substrate manufacturing.